Light-emitting diodes are semiconductor illumination devices formed by using semiconductor materials. The light-emitting diodes are one kind of minute solid-state light source that can transform electrical energy into light energy. The semiconductor have many advantages, including small volume, a low driving voltage, rapid response speed, resistance to shock, and long life, and also can meet the light, thin, and miniaturization needs of various apparatuses. Light-emitting diodes have thus have become very popular electric products in daily life.
In the fabrication of light-emitting diodes, III-nitride-based semiconductors, such as GaN, AlGaN, InGaN and AlInGaN, are common. Usually, epitaxial structures of most of the light-emitting devices made of the III-nitride-based semiconductors are grown on an electrically insulating sapphire substrate, which is different from other light-emitting devices utilizing conductive substrates. The sapphire substrate is an insulator, so an electrode can be directly formed on the sapphire substrate. Electrodes have to be formed to contact respectively a p-type semiconductor layer and an n-type semiconductor layer directly, so that the light-emitting devices of the aforementioned type can be completed.
FIG. 1 illustrates a cross-sectional view of a conventional light-emitting diode. In the fabrication of the light-emitting diode, a buffer layer 102 is firstly formed on a transparent substrate 100 by deposition. The substrate 100 is typically made of sapphire. An n-type semiconductor layer 104 is epitaxially formed on the buffer layer 102, in which the n-type semiconductor layer 104 is typically made of III-nitrides. Next, an active layer 106 is epitaxially formed on the n-type semiconductor layer 104, in which the active layer 106 is typically a multiple quantum well (MQW) structure. A p-type semiconductor layer 108 is epitaxially grown on the active layer 106, in which the p-type semiconductor layer 108 is typically made of III-nitrides. An illuminant epitaxial structure is composed of the n-type semiconductor layer 104, the active layer 106 and the p-type semiconductor layer 108.
The substrate 100 is made of sapphire, an insulator, so contact electrodes of the light-emitting diode have to be formed to contact respectively the p-type semiconductor layer 108 and the n-type semiconductor layer 104 directly, and cannot be directly formed on the substrate 100 made of sapphire. Accordingly, after the illuminant epitaxial structure is formed, a definition step is performed by photolithography and etching to remove a portion of the active layer 106, a portion of the p-type semiconductor layer 108 and a portion of the n-type semiconductor layer 104, so as to expose a portion of the n-type semiconductor layer 104. Then, an electrode 110 and an electrode 112 are respectively formed on the p-type semiconductor layer 108 and the exposed portion of the n-type semiconductor layer 104 is formed directly by thermal evaporation, E-beam evaporation or ion-sputtering, so that the fabrication of the light-emitting diode is completed.